This invention relates to a diaphragm of an electroacoustic transducer, such as a microphone, and more particularly a diaphragm having a uniform vibration characteristic over a wide frequency bandwidth and improved durability and reliability, and a method of manufacturing such a diaphragm.
The frequency bandwidth of such electroacoustic transducer, such as a loudspeaker, is generally determined by the material of the diaphragm so that it is necessary to use material having a lightweight, a large Young's modulus to density ratio E/.rho. and a large flexural rigidity E.multidot.I (where E represents Young's modulus, .rho. density, and I secondary moment of section) and having a suitable value of internal loss tan .delta.. More particularly, the higher is the Young's modulus to density ratio E/.rho., the larger is the high tone limit frequency (fh) which broadens the piston motion range. Accordingly, the frequency bandwidth of the loudspeaker is widened. Furthermore, as the flexural rigidity E.multidot.I is increased, strain is decreased, and when the internal loss tan .delta. has a suitable value, the Q-value of the partial resonance of the diaphragm is decreased to a suitable value making it difficult for partial vibration to occur thus, making flat the characteristic (there is no coloring of the reproduced sound). In the design of a diaphragm, as it is necessary to reproduce a transient waveform signal containing high frequency components (this is true in most musical signals), a wide bandwidth characteristic is desired. For the reasons described above, it is important to select a suitable material. In recent years, beryllium having higher Young's modulus to density ratio E/.rho. than aluminum alloys, and titanium alloys have been used.
However, beryllium is not only expensive but also brittle and difficult to be worked. Accordingly, recently a composite material constituted by fibers having a high Young's modulus to density ratio, for example carbon fibers, and a matrix, such as epoxy resin which combines the fibers, has been used. When compared with the prior art light alloys described above, such composite material has a larger Young's modulus E and a smaller density .rho. so that its Young's modulus to density ratio E/.rho. is large and it has a desirable frequency characteristic over a wide frequency bandwidth. Moreover, the composite material can be facilely manufactured at a low cost.
According to a prior art method, a dome-shaped diaphragm 2, as shown in FIG. 1a, was prepared by cutting a flat sheet 1 formed by aligning in the direction A a number of fibers having a high Young's modulus to density ratio (as shown in FIG. 1b) into a configuration corresponding to a development of a dome-shaped diaphragm such that the fibers intersect at right angles with the bottom circle of the dome, and then bonding the cut portions with a matrix material into a dome as shown in FIG. 1a. Consequently, the bonding force at the joints 3 is not sufficient so that these joints would be broken under a high output vibration condition, thus decreasing the durability and reliability of the resulting diaphragm.
More particularly, when the fiber sheet 1 is cut into a saw toothed shape and the side edges of adjacent teeth are bonded together the ends of some of the fibers would be abutted. In such a case, the resin acting as the bonding agent does not enter between the abutting ends, thus decreasing the bonding force. To obtain sufficient bonding strength it is desirable that the ends of the fibers of adjacent teeth be staggered. Moreover, when the ends of the fibers are abutted, air layers would be formed therebetween, thus decreasing the bonding strength.